The lamp, basically, has a glass tube bent once or several times which consists of several straight, parallel longitudinal tube sections and at least one transverse tube section connecting individual longitudinal tube sections, with the two outer ends of the discharge tube being sealed and retaining electrode seals.
This structure provides a relatively compact low-pressure mercury vapor discharge light source. The sealed outer ends of the discharge tube may be provided with a base which permits the lamp to be used in a suitable fixture.
Low-pressure discharge lamps are known in which the tubular discharge envelope is simple and bent to U-shape (e.g. from the German Patent DE-PS No. 830,219, Japanese Utility Model Publication No. 36-27 470 and German Patent Disclosure Document DE-OS No. 28 35 183 to which U.S. Pat. No. 4,353,007 corresponds). Discharge lamps are also known in which the discharge tube is bent or folded several times (e.g. from the German Patent Disclosure Document DE-OS No. 29 42 846 and the German Patent Disclosure Document DE-OS No. 30 05 017). In some lamp constructions, the bent discharge envelope is surrounded by an outer bulb.
A common feature of all of these known lamps is that the individual bends of the discharge tube--between the longitudinal tube sections and the respective connecting transverse tube sections--extend at the outwardly facing tube surfaces in a uniform, gentle curve with a relatively large bending radius. The cross section of the discharge tube remains substantially equal over the individual tube sections, including the transition sections. The outer generating line of the respective transverse tube sections also is arcuate until it merges into the longitudinal tube sections.
In lamps of this type, above all in lamps of compact design, problems may occur with respect to maintaining the optimal mercury vapor pressure in the discharge envelope. It is well known that the mercury vapor pressure determines optimal conversion of the electrical power supplied to the lamp into radiation output emitted by the discharge; vapor pressure, in turn, is determined by the coldest spot or zone of the envelope wall.
In the heretofore known lamps, the coldest spot is located in most cases at the U-shaped, bent transverse tube sections which are located furthest from the electrode ends which are hotter. Especially in the case of the compact lamp constructions, which may in addition be provided with an outer bulb, the normal wall temperature, which is approximately uniform over the respective transverse tube section appearing in the arcuate transverse tube sections is too high. To compensate, artificial cold spots are created at or adjacent these tube sections.
It has therefore been proposed, for example, in a lamp having a discharge tube bent three times to U-shape and an outer bulb (see German Patent Disclosure Document DE-OS No. 29 42 846) to provide a bulge at one of the transverse tube sections which projects through a planar member made, e.g. of glass, which extends transversely to the longitudinal tube sections. The planar member keeps the bulge at a relatively low temperature.
It has been proposed (see German Patent Disclosure Document DE-OS No. 30 11 382 to which U.S. Pat. No. 4,374,340 corresponds) to construct a low-pressure mercury vapor discharge lamp in which several parallel, staight, tubular parts are connected by tubular coupling members extending transversely to the said parts. These discharge envelopes are manufactured as individual tube members which are not interconnected originally. The coupling members have a smaller diameter than the longitudinal tube ports. The tubular coupling members extend at a certain distance from the respective adjacent ends of the longitudinal tube parts. The discharge does not reach the closed tube ends, so that they form the coldest spots of the discharge envelope.